As the economic idea of the green environmental protection and low carbon is rooted in the hearts of people in the world, operators' requirements for reducing power consumption of wireless communication systems become higher and higher. In a wireless communication system, a radio frequency power amplifier (power amplifier for short) in a base station device is one of core modules of the whole system. An important index of the radio frequency power amplifier is the efficiency of the power amplifier. Data analysis shows that energy consumption of a power amplifier occupies about 60% of total energy consumption in the whole base station device, thus improving the efficiency of the power amplifier becomes the most effective means to reduce power consumption of the base station device and decrease operation expense (OPEX) of operators. Therefore, facing increasingly severe market competition of wireless communications, high efficient radio frequency power amplification technology has become one of competitive focuses of wireless communication industry.
Doherty power amplifier, which is a most widely used high efficient power amplification technology in wireless communication systems currently, is invented by an American electronic engineer named William H. Doherty in 1936. However, in the following about 30 years, people's attention had been diverted. Until the late 1960s, with the development of communication technologies, especially satellite communications, efficiency and linearity issues of power amplifiers were re-proposed in new historical occasions, and Doherty amplifiers came back to people's horizon and were widely applied in communication and broadcast systems in 1970s. At present, the Doherty power amplification technology, used in conjunction with the digital pre-distortion (DPD) technology, has become a mainstream form of high efficient power amplifiers of base stations in wireless communication systems.
The basic idea of the Doherty power amplifier is active load pull. A traditional power amplifier is shown in FIG. 1, which mainly comprises driver stage amplifiers (Dr1 . . . Drn in the figure), a power distributing circuit (D in the figure), a carrier amplifier (C in the figure, also called a main power amplifier), a peak amplifier (P in the figure, also called an auxiliary power amplifier), a power combining circuit (Combiner in the figure), etc. The carrier amplifier operates as a class B or class AB amplifier, and the peak amplifier operates as a class C amplifier. The two amplifiers bear different input signal power and operate in their respective saturation regions as much as possible, thereby guaranteeing that the whole power amplifier maintains higher efficiency in an input signal power range as large as possible, and guaranteeing certain linearity at the same time.
The Doherty power amplifier mainly includes the following three types of operating states.
Small signal area: when input signals are relatively small, the peak amplifier is in an off state, and the carrier amplifier operates as a class AB amplifier, at this point, the carrier amplifier operates in a maximum efficiency matching state.
Load modulation area: when the input signals are increased to a certain extent, a gradual transition of the carrier amplifier from an amplification area to a saturation area appears, and a gradual transition of the peak amplifier from a cut-off area to the amplification area appears, at this point, loads of both the carrier amplifier and the peak amplifier are not stable, and their load impedances vary with variation in power.
Saturation area: with continuous increase in input signals, both the carrier amplifier and the peak amplifier will operate in a saturation state finally and correspond to a load of 50 Ω, and their output powers are added.